1. Field of the Invention
The present invention relates to an annular type combustor applied to a gas turbine.
2. Description of the Prior Art
One example of an annular type gas turbine combustor, which is known in the prior art, is described with reference to FIGS. 4 to 6. FIG. 4 is a schematic cross sectional view of a combustor, taken on a vertical plane including a turbine central axis, which shows only an upper half portion of the combustor above the turbine central axis. That is, the annular type combustor forms a combustor which is concentric with respect to the turbine central axis and forms an annulus (like a doughnut shape) surrounding the turbine central axis, together with a lower half portion (not shown) of the combustor turbine below the central axis.
In FIG. 4, numeral 1 designates a diffuser, numeral 2 designates a diffuser case, numeral 3 designates a fuel injector, numeral 4 designates a swirler, numeral 5 designates a liner, numeral 6 designates a dilution hole, numeral 7 designates a cooling hole and numeral 8 designates a rotating ring. In a combustor as so constructed, air passes through the diffuser 1 and enters a combustion region surrounded by the liner 5 via the swirler 4, the dilution hole 6, the cooling hole 7, etc.
In the combustor, the level of NO.sub.x included in an emission gas is influenced by a ratio of fuel to air passing through the swirler 4 (hereinafter referred to as "fuel-air ratio"), and generally as the fuel-air ratio is lowered, the NO.sub.x level becomes lower. Thus, in order to attain a low NO.sub.x level under every operating condition of a gas turbine, there is needed a mechanism for regulating the fuel-air ratio.
In the prior art combustor, therefore, a rotary type flow regulating mechanism, employing the rotating ring 8, at the swirler 4, is provided to change the cross sectional area of the swirler and thus regulate the fuel-air ratio. As shown in FIGS. 5 and 5, the rotating ring 8 has approximately the same structure as the swirler 4 and is constructed with swirling blades, each having a certain thickness. Air flow is regulated by the rotating ring 8 which is rotated so that a relative phase to the swirler 4 is changed.
That is, if the air passage of the rotating ring 8 coincides with the air passage of the swirler 4 at a contact plane, the opening cross sectional area becomes a maximum, and as the relative phase is caused to deviate from this state, as shown in FIG. 6, the opening cross sectional area can be reduced corresponding to the amount of such deviation. According to the change of the opening cross sectional area, the flow rate of air passing through the swirler 4 is increased or decreased with the result that the fuel-air ratio can be regulated.
In the prior art combustor, the rotating ring which is a movable portion for regulating the fuel-air ratio is disposed at a position which is easily influenced by flames, and thus there exists a problem in that the rotating ring is heated to temperatures which will cause the ring to lose its durability and reliability. Also, in such an arrangement, there is a problem in that a complicated mechanism for the movable portion is needed as a countermeasure for thermal expansion etc.